Sorry, I meant Electrical or mechanical. I have a 1998 Lariat Supercab 4x4 and my 4wd suddenly won't do anything. No lights & no noise of any kind. I do have the lights in the dash momentarily when you first start it but of coarse they go out. All fuses show good except #20 in JB and I have read on other posts that it's for manual tranny only. I have no power to the dash switch (electronic shift on the fly) and I'm beginning to think it works more in a ground directional manner.
Can anyone tell me the line of action that begins once you turn the ****? Where does it go from there? Any help will be greatly appreciated!!!!!!!

 

The wires to the 4WD selector switch are signal through a resistor in the switch and back to the GEM, no power ( might show as ground on the signal + ? ).

the GEM selects what you want to do based upon the resistance that is returned on the signal return line ( one of those wires ).

Best to start with does the vacuum diaphragm move when you select 4-Hi.

it is on the front axle ( this is a members picture of one that is broken, which could be another issue ).



If the Vacuum diaphragm does not move, it could be a vacuum issue or a problem with the shift solenoids on the firewall.

There are the two lines that operate the diaphragm on the axle. Facing the truck, the right solenoid (pink & black lines) is activated to disconnect the axle and go back into 2WD. The left solenoid (blue & white lines) lock the axle into 4WD.
***Note that the lines are reversed on the diaphragm in this photo, thanks to FORDS incorrect vacuum diagram***


Some members have reported an issue with not being able to hold vacuum on the line, and the vacuum reservoir behind the battery box was cracked.

Check the vacuum diaphragm to see if it holds vacuum.

The blue line should have vacuum on it when it's in 4WD and the pink line should when it's in 2WD.
There are two solenoids on the right cowl to control the vacuum.

If there is vacuum on the black & white supply lines to the solenoids and nothing comes out on the pink/blue lines then a solenoid is bad.
The solenoids have constant 12V supplied to them and are activated by ground through the GEM depending on the switch position ( the resistance level returned to the GEM ).

Also seem to recall one member had a cracked T ( in the vacuum diagram ) where it supplied vacuum to the solenoids. Not sure who, but I seem to recall it on the site here.

 

Thanks SSULLY!!! I've never worked with any meters but I picked up a multimeter today. Could you please walk me thru the checking of the switch? What do I set it on? What wires do I check? Do I leave the switch hooked up? Key on or off? Sorry SSCULLY, but after many views you were the only person that offered any help so I'm sorta depending on you! Thanks again & please be patient!!

 

If you want to start with checking the switch, it is the 2 dark blue wires to it.
Find which wires they are, and unplug from the truck

Set the meter to ohms, and touch a meter lead on each pin in the connector.
This can be a bit of a pain - might need to use a short section of 20 AWG solid wire or straight pins ( clothes type ) with the alligator clips, and touch the pins after moving the switch to the position to test. Don't hold the meter tips to the pins with your fingers, that adds your bodies resistance to the test, which can give you screwy number.

When you change from one mode to the next, you will get the resistance in the chart above.

I have not seen a switch go bad yet, 90% of the issues are with the vacuum operation ( tear in the shift diaphragm, cracked vacuum lines or reservoir box one case the plastic T to the shift solenoids was cracked ).

The remaining 10% of the issues fall under shift solenoid issues, transfer case or GEM items.

Start with checking if the shift diaphragm moves when selecting in and out of 4WD ( 4H or 4L ).

If you have a vacuum tester like a Mighty Vac, you can use this to connect to the shift diaphragm and apply vacuum to see if it moves, and then check the gauge to make sure there is no bleed down on the vacuum pressure.

The switch really does not have a lot going on ( it is just different resistors ) so not much in terms of load to make the resistors go bad, where vacuum lines get brittle and crack over time, and at 15 years, this could be the case.
- Not to say it cannot be the switch, I just put it so low on the list.

The other items after checking the vacuum operation is the transfer case shift plates shift motor and the connectors for them
- Tells the GEM where the transfer case it at and where to move it to.

Your call if you want to start with the switch.

The other item, have you had a lot of rain as of late ?
- The GEM can get wet if the windshield is leaking and this causes strange things to happen. Usually other things happen that are more noticeable.

 

Thanks SSCULLY, I'll try that.
Ok, I checked the switch so now I just have to wait until I find someone to turn the switch while I'm under the truck to see if vacuum is working the actuator. Guess I could go ahead & put it in 4wd & check to see if TC is shifting. Thanks again SSCULLY!! Thanks to you I have learned how to use a Multimeter! Will get back to you after I run more tests.

 

Could do that solo, and get it all at one time.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't turn it is a front axle engagement problem.

- If the front drive shaft does not turn :
With the truck running ( with front wheel back on the ground ), switch from 2WD to 4WD and back again, the vacuum shift motor should move the lever on the front axle. Just note which position it is in before you walk back to flip the switch.

-- If the vacuum shift motor does not move:
1.Check for vacuum on the lines while shifting between modes. Pink vacuum line is 2WD mode, Light Blue vacuum line is 4WD.
2.Check the fuses for the 4WD system and the solenoids.
3.Check the vacuum lines from the vacuum shift motor to the solenoids on the firewall.
4.Check the vacuum lines from the solenoids to the vacuum reservoir box behind the battery.
5.Check the vacuum reservoir box for cracks.
6.Typical solution is the vacuum lines ( to the solenoids or to the vacuum shift motor ) are cracked or the vacuum shift motor has a cracked diaphragm and/ or has water in it.
7.Could be the shift plate contacts in the transfer case are corroded / not making good contact, and not activating the solenoids.

- If the front drive shaft turns:
For ESOF system
-- Check the fuses for the transfer case, engine compartment fuse panel, 17 & Cab fuse panel fuses 13 and 23 ( verify with owners manual, 97-97 & 99-03 MY can be slightly different on some items ).
-- Try hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ) to see if the transfer case shift motor is hung up .
-- Check the transfer case connector, to make sure the connector pins are clean and making good contact.

That should get you down to a specific item that is wrong ( think I covered all areas )..

Let us know what happens.

 

OK, here's where I'm at; I started the truck, put it in 4high & waited 3-5 mins. Put truck in park with it still in 4h, shut off truck & crawled under. Front shaft turns freely. I hit shift motor w rubber mallet & nothing...it's dark now so tomorrow I'll crawl under & check connection. Does the vacuum have anythig to do with shift motor? Also, I checked fuse #13 &# 23 both good. My 1998 manual shows # 20 to be PCM, GEM & CTM and there is no power to that fuse. I think I saw on a post somewhere that # 20 was for manual trans only & mine is auto. Also fuse 13 shows to be for "rear anti lock brake system (RABS) Module, Brake on/off (BOO) switch, Brake Pressure Switch.

 

Engine Compartment
17 - Transfer case shoft relay module.

Cab fuse panel
13 - is the Brake on / off fuse, but this is included as part of what is needed to shift into 4L.
23 - Transfer case Eletric clutch relay, 4x2 center axle disconnect solenoid & 4x4 center axle disconnect solenoid.

If the front drive shaft spins freely, there is a problem with the transfer case itself.

The connector could be corroded or not getting power or the shift motor not working anymore. After checking the connector, and that +VDC is making it to the transfer case ( wire color below ) that having someone turn the switch while hitting the transfer case with a dead blow mallet might shake it free if it is just stuck.
I seem to recall one member that had a broken wire in the passenger side door sill, that was the power to the shift motor - dark green w/ light green stripe wire. I am old, so I could be off on that, the thread is on the site here somewhere ( maybe search on here for 4WD issues, or try transfer case as the search term ?? )

The Dark Green w/ light green stripe wire might look black, take something under there with you, to clean the wires on the transfer case connector.
If you test from a known good ground to the Dark Green w/ light green stripe wire with the meter in the DC volt setting, that should give you an idea on where to look,

The Dark Green w/ light green stripe wire is from the engine compartment fuse panel, fuse # 17.

 

OK, here's where I'm at. I crawled under the truck with key on, unplugged connector to TC (could not locate green wire) had my grandson move switch to 4 H repeatedly as I checked every wire with test light grounded to cross member(not rusted) & found no fire to any wire inside of connector. I tried each wire minimum of 3 times with 5 second intervals. What do you suggest? Thanks in advance SSCULLY for all your help!!!

While I was waiting on a reply, since I had my Grandson here to help, I started my truck & crawled back under & had him move switch to 4 H. The acuator on the third member never moved.... can this be connected to not having fire to TC?

P.S. I tested & both shift sols. are hot on one side. To make it even more confusing, I jacked up the front of the truck (with it in 2 H & both hubs seem to be locked in. When I turn the wheel on passenger side the axle turns with it & the same results when I turn drivers side but the front drive shaft doesn't turn. (turning either tire does not make the other one turn) What do you make of all this?

 

You are looking for a dark green w/ light green stripe wire, not a green wire.

You did not have a fire on the wires, or you did not get + VDC ?

The ground you are using, are you sure it is ground ?
- no rust does not make it ground.

What is the actuator on the third member ?

The solenoids are hot at all times on one side, the ground is switched to activate them ( that is normal & as designed ).

Sounds like the front axle is locked in, but the transfer case is not activating.

This is either an issue with the wiring to the transfer case or the shift motor in the transfer case having an issue.

Without the transfer case signalling the GEM, the front axle will not unlock via the shift solenoids.

If you want to move it back to unlock, you can apply vacuum to the shift diaphragm to get it to unlock the front axle. If the shift motor in the transfer case is stuck, it might shift back to lock again.
- You might need to apply vacuum to the shift diaphragm and cap the vacuum lines for the mean time.

 

Let's try this; I went back under the truck with a ground wire from the batt. connected to my testlight & also took my multimeter. I thought using the crossmember for ground was ok because I have a frame ground to batt., anyway, I still found no green, green / stipe or any shade of green on any wire so I remooved the TC connector & checked each pin first with testlight & then with multimeter & still found no fire or +VDC. I rechecked the fuses including #17 in EC.

Next I removed the batt & batt box to inspect vacuum lines & resivoir with all looking good. Checking both SSs had vacuum in but only the Right one (facing truck) had vacuum leading to shifting diaphram. The only SS that clicks is the right one every time the ign is turned off or on. I can reverse the vacuum lines & the axle locks & turns front driveshaft. Changing them back unlocks the axles but not the HUBS!!
When I test the wires going to the SSs, there is ground going to the right one no matter the switch position but can get no ground to left SS no matter what.

 

If ground is stuck on one of the shift solenoids, sounds like the plates in the TC or the shift motor are stuck or not getting power.

Here are the diagrams, so you can see what I am talking about.









There is a chance it is caused by the GEM, but going most likely to least, I would put the GEM at the bottom without other symptoms.

 

Thank you SSCULLY for the info. I'm not much at reading diagrams (schematics) but I'm leaning towards the Transfer Case Shift Relay Module. Can you tell me where it's located please?

PS I know that you went to alot of trouble to locate & post the diagrams for me & I WANT YOU TO KNOW i APPRECIATE the trouble you went thru!

 

Don't have my 97 EVTM location diagrams handy, but the 2001 EVTM shows this as in the dash, about straight up from the OBD connector.
- Almost behind the center ( center right ) side of the instrument panel / gauges. straight up from the flasher / HVAC blower relay combo.

This is from a 2001, so it is a bit risky to rely on it 100%
- There are several changes from 97/98 to 99 to 00/03 series.

The connector shows ( in the 2001 ) as a black 10 pin connector that is a row of 4 ( on the tab side ) and 6 on the other side.

That is all I have until I can get to my 97/98 EVTM.

 

Hey SSCULLY, you still around? Sorry it took so long to get back to you but a quack of a Surgeon butchered my neck up. I replaced the TC relay but could of never found it w/o the diagrams you sent me. Just my luck...still won't work!!!! Any suggestions?